Orthogonal Frequency-Division Multiplexing (OFDM) technology is a key underlying component of the fourth-generation wireless network technologies known as Long-Term Evolution (LTE) and developed by the 3rd-Generation Partnership Project (3GPP). As is well known to those skilled in the art, OFDM is a digital multi-carrier modulation scheme employing a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is separately modulated using conventional modulation techniques and channel coding schemes. In particular, 3GPP has specified Orthogonal Frequency Division Multiple Access (OFDMA) for the downlink transmissions from the base station to a mobile terminal, and single carrier frequency division multiple access (SC-FDMA) for uplink transmissions from a mobile terminal to a base station. Both multiple access schemes permit the available sub-carriers to be allocated among several users.
SC-FDMA technology employs specially formed OFDM signals, and is therefore often called “pre-coded OFDM” or Discrete-Fourier-Transform (DFT)-spread OFDM. Although similar in many respects to conventional OFDMA technology, SC-FDMA signals offer a reduced peak-to-average power ratio (PAPR) compared to OFDMA signals, thus allowing transmitter power amplifiers to be operated more efficiently. This in turn facilitates more efficient usage of a mobile terminal's limited battery resources. SC-FDMA is described more fully in Myung, et al., “Single Carrier FDMA for Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine, vol. 1, no. 3, September 2006, pp. 30-38.
The basic LTE physical resource, for both uplink and downlink communications, can be viewed as a time-frequency grid. This concept is illustrated in FIG. 1, which shows a number of so-called subcarriers in the frequency domain, at a frequency spacing of Δf, divided into OFDM symbol intervals in the time domain. Each grid element 12 is called a resource element, and corresponds to one subcarrier during one OFDM symbol interval, on a given antenna port. One of the unique aspects of OFDM is that each symbol 14 begins with a cyclic prefix 16, which is essentially a reproduction of the last portion of the symbol 14 affixed to the beginning. This feature minimizes problems from multipath, over a wide range of radio signal environments.
In the time domain, LTE downlink and uplink transmissions are organized into radio frames of ten milliseconds each, each radio frame consisting of ten equally-sized subframes of one millisecond duration. This is illustrated in FIG. 2, where an LTE signal 20 includes several frames 22, each of which is divided into ten subframes 24. Not shown in FIG. 2 is that each subframe 24 is further divided into two slots, each of which is 0.5 milliseconds long.
LTE link resources are organized into “resource blocks,” defined as time-frequency blocks with a duration of 0.5 milliseconds, corresponding to one slot, and encompassing a bandwidth of 180 kHz, corresponding to 12 contiguous sub-carriers with a spacing of 15 kHz. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Two time-consecutive resource blocks represent a resource block pair, and correspond to the time interval upon which scheduling operates. Of course, the exact definition of a resource block may vary between LTE and similar systems, and the inventive methods and apparatus described herein are not limited to the numbers used herein.
In general, however, resource blocks may be dynamically assigned to mobile terminals, and may be assigned independently for the uplink and the downlink. Depending on a mobile terminal's data throughput needs, the system resources allocated to it may be increased by allocating resource blocks across several sub-frames, or across several frequency blocks, or both. Thus, the instantaneous bandwidth allocated to a mobile terminal in a scheduling process may be dynamically adapted to respond to changing conditions.
For scheduling of downlink and uplink data to and from the mobile terminal, the base station transmits control information in each subframe. This control information identifies the mobile terminals to which data is targeted and the resource blocks, in the current downlink subframe, that are carrying the data for each terminal. The first one, two, three, or four OFDM symbols in each subframe are used to carry this control signaling. In FIG. 3, a downlink subframe 30 is shown, with three OFDM symbols allocated to control region 32. The control region 32 consists primarily of control data elements 32, but also includes a number of reference symbols 34, used by the receiving station to measure channel conditions. These reference symbols 34 are interspersed at pre-determined locations throughout the control region 32 and the rest of the subframe 30.
Uplink user data is carried on the Physical Uplink Shared Channel (PUSCH), which is defined by the configured uplink transmission bandwidth and the frequency hopping pattern signaled to the mobile terminal, if any. The Physical Uplink Control Channel (PUCCH) carries uplink control information, such as CQI reports and ACK/NACK information for data packets received in the downlink. The PUCCH is transmitted on a reserved frequency region in the uplink, identified to the mobile terminal by higher-layer signaling.
Two types of reference signals are used in the uplink. The demodulation reference signal (DRS) is used by the eNodeB receiver for channel estimation in order to demodulate control and data channels. The DRS occupies the fourth symbol in each slot (for normal cyclic prefix) and spans the same bandwidth as the allocated uplink data. The sounding reference signal (SRS) provides uplink channel quality information for use by the eNodeB in scheduling decisions. The UE sends a sounding reference signal in parts of the configured transmission bandwidth where no uplink data transmission is available. The SRS is transmitted in the last symbol of the subframe. The specific configuration of the sounding signal, in terms of its bandwidth, duration, and periodicity, is provided to the mobile terminal via higher-layer signaling.
Release 8 of the LTE specifications has recently been standardized. Among its features is support for bandwidths up to 20 MHz. However, in order to meet the IMT-Advanced requirements for very high data rates, 3GPP has initiated work on LTE Release 10 specification. One objective of Release 10 is the support of bandwidths larger than 20 MHz. The Release 10 and later releases of the LTE specification are sometimes referred to as “LTE-Advanced.”
One important requirement on LTE Release 10 is to assure backward compatibility with LTE Release 8, including with respect to spectrum compatibility. This means that an LTE Release 10 carrier signal, which might be wider than 20 MHz, should appear to a Release 8 mobile terminal in that event as several smaller LTE carriers. This concept is known as carrier aggregation (CA), or “multi-carrier” operation, and each of these smaller LTE carriers is often referred to as a component carrier (CC).
For some time following the initial deployment of LTE Release 10 networks, it can be expected that there will be a relatively small number of LTE Release 10-capable terminals, compared to so-called legacy terminals that are designed to Release 8 of the specifications. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e., that it is possible to implement wide carriers, so that Release 10 mobile terminals can exploit the very high data rates, but in such a way that legacy terminals can be scheduled in each part of the wideband LTE Release 10 carrier. With carrier aggregation, an LTE Release 10 terminal can receive multiple component carriers, where each component carrier can have the same structure as a Release 8 carrier.
The carrier aggregation concept is illustrated in FIG. 4, where five component carriers 40 are illustrated, with respective component carrier bandwidths of f1, f2, f3, f4, f5. In this case, the total bandwidth available to a Release 10 mobile terminal is the sum of the component carrier bandwidths. Release 8 mobile terminals can be scheduled to use resources in any one of the component carriers. Note that while the component carriers in FIG. 4 are illustrated as contiguous (i.e., immediately adjacent to one another in frequency), aggregated carrier configurations where one or more of the component carriers is not adjacent to the others are also possible.
Furthermore, the number of aggregated component carriers, as well as the bandwidth for each individual component carrier, may be different for uplink and downlink operation. A symmetric configuration refers to the case where the number of component carriers in downlink and uplink is the same, while an asymmetric configuration refers to the case where the number of component carriers is different. It is important to note that the number of component carriers configured in a given cell may be different from the number of component carriers “seen” by a terminal. For instance, a particular terminal may support more downlink component carriers than uplink component carriers, for example, even though the cell is configured with the same number for uplink and downlink.
During initial access to the network, a LTE Release 10 terminal behaves similarly to a LTE Release 8 terminal. Upon successful connection to the network using a single component carrier for each of the uplink and downlink, a terminal may—depending on its own capabilities and the network—be configured with additional component carriers in either or both of the uplink and downlink. Configuration of the carriers is performed with Radio Resource Control (RRC) signaling.
Due to the heavy signaling and rather slow speed of RRC signaling, a terminal may be configured to operate with multiple component carriers even though not all of them are continuously used. If a terminal is configured on multiple component carriers, this would suggest that it has to monitor all downlink component carriers for the Physical Downlink Control Channel (PDCCH) and the Physical Downlink Shared Channel (PDSCH). This implies the use of a wider receiver bandwidth, higher sampling rates, and so on, at all times, potentially resulting in higher power consumption than necessary.
To mitigate the above problems, LTE Release 10 supports activation of component carriers, in addition to configuration of component carriers. With this approach, a mobile terminal continuously monitors only component carriers that are both configured and activated. Since the activation process is based on Medium Access Control (MAC) control elements—which are much faster than RRC signaling—an activation/de-activation process can quickly adjust the number of activated component carriers to match the number that are required to fulfill the current data rate needs. Upon arrival of large data amounts, multiple component carriers are activated, used for data transmission, and then quickly de-activated if no longer needed. All but one component carrier, the downlink primary component carrier (DL PCC), can be de-activated. Activation therefore provides the possibility to keep multiple component carriers configured, for activation on an as-needed basis. Most of the time, a terminal would have only one or a very few component carriers activated, resulting in a lower reception bandwidth and lower battery consumption.